In electrographic recording an electrostatic charge is deposited directly on an insulating or recording medium in a pattern or image corresponding to the charges applied to the pin electrodes. Generally, the styli are in contact with the recording medium and an electrostatic charge is deposited on the recording medium by the electric field established between the styli and a ground plate conventionally called a platen located at the opposite side of the record medium. The deposition of a charge on the recording medium leaves marks or dots thereon which when properly combined form the data to be printed. The insulating medium may be a plastic coated paper, pre-dried paper or any other dielectric sheet capable of maintaining a high resistance and holding an electrostatic charge for a useful period of time. In order to form a visible rendering of the electrostatic image it is necessary to develop the image with electroscopic powder or toner. A toner is selected so that it will be charged to a polarity opposite to that of the electrostatic image. When this toner is deposited upon the charged surface, the image exerts a force of attraction on the toner and retains the toner in the charged areas. Subsequently the toner may be fused into the recording medium making a permanent record.
In most styli printing apparatus the styli have limited life due to the wear of the pin electrodes as they are swept across the recording medium. Consequently, the styli must be replaced periodically adding to the expense of operation of the printer. Many attempts have been made to obtain styli which have longer life in order to reduce frequent replacement. However, these attempts have either resulted in complete failure or in increased fabrication costs.
One way of obtaining long lasting styli is to make the conductors as hard as possible. This approach creates problems of fabricaton because hard materials such as tungsten, tungsten carbide, nickel-tungsten, hard chromium, stainless steel or beryllium-copper are difficult to place on etched copper conductors. Attempts to do so result in cracking of the plating on the writing tips or in shorting of the electrode conductors because of the close spacing of the etched pattern.
Accordingly, it is an object of this invention to provide a stylus for a printing apparatus which has long life, is inexpensive and easy to fabricate.
It is another object of this invention to increase the life of a stylus employed in a printing apparatus by a novel method of manufacture involving overlaying the writing edge with a wear resistant or hard material.
It is yet another object of this invention to increase the life of a stylus by forming the writing edge on a wear resistant or hard substrate.